Add msys2 library dependency tag in gem metadata.

RubyInstaller2 supports metadata tags for installation of dependent
MSYS2/MINGW libraries. The glut source gem requires the freeglut
package to be installed on the system, which the gem installer takes
care about, when this tag is set.

The feature is documented here:
https://github.com/oneclick/rubyinstaller2/wiki/For-gem-developers#msys2-library-dependency

Rakefile

@@ -38,6 +38,7 @@ hoe = Hoe.spec 'glut' do
   self.spec_extras = {
     :extensions            => %w[ext/glut/extconf.rb],
     :required_ruby_version => '>= 1.9.2',
+    :metadata              => {'msys2_mingw_dependencies' => 'freeglut'},
   }
 end
 
@@ -49,6 +50,10 @@ Rake::ExtensionTask.new 'glut', hoe.spec do |ext|
   ext.cross_config_options += [
     "--enable-win32-cross",
   ]
+  ext.cross_compiling do |spec|
+    # The fat binary gem doesn't depend on the freeglut package, since it bundles the library.
+    spec.metadata.delete('msys2_mingw_dependencies')
+  end
 end